1. Field of the Invention
The present invention relates to a multicast communication method to be performed in a CDMA (Code Division Multiple Access) mobile communication system.
2. Description of the Related Art
As a conventional multicast communication method in the CDMA mobile communication system, Japanese Patent Laid-Open Hei 10 (1998)-107770 discloses a multicast communication method in which service negotiations for the multicast communication is performed in a calling procedure between a mobile subscriber (abbreviated as MS, hereinafter) and a base station system(abbreviated as BSS, hereinafter) for the MS obtaining a spread code to be used for each multicast communication method.
Alternatively, in a plurality of BSSs, each multicast service data has been transmitted by using a broadcast channel of each BSS without any linkage to one another.
However, the following problems were inherent in the foregoing conventional multicast communication method in the CDMA mobile communication system.
(1) Regarding a system where a spread code is allocated for each multicast communication method by service negotiations in a calling procedure between the MS and the BSS, since a registration operation for the spread code is executed for each multicast service in MS that receives information, processing becomes complex. Thus, the system is not suitable for the multicast communication method of advertising use where information is simultaneously transmitted to a large number of MS. In addition, since the spread code must be fixed for each multicast communication method to be notified, dynamic use of limited number of spread codes is not allowed. Thus, this system lacks efficiency.
(2) Regarding a system where each multicast communication data is transmitted by each BSS by using respective broadcast channels, since the position of MS cannot be specified when a multicast operation is performed, transmission power in each BSS must be set to a maximum in order to enable possible MS located in the vicinity of a boundary to receive a signal sent from BSS. But if signal for data is transmitted by an output more than necessary, noises are generated in other MS. Consequently, a capacity of the entire system is reduced.
The present invention was made in order to solve the foregoing problems inherent in the prior art, and it is an object of the invention to provide a multicast communication method in a CDMA mobile communication system, which is capable of reducing transmission power in BSS and performing an efficient multicast operation for a large number of MS.
In order to achieve the foregoing object, in accordance with an aspect of the present invention, a multicast communication method in a CDMA mobile communication system, which comprises a plurality of mobile subscribers, a plurality of base station systems connected to the mobile subscribers by radio channels, a plurality of mobile switching centers connected to the base station systems and a multicast communication server connected to the mobile switching centers for providing information to be distributed to the mobile subscribers, the method is characterized in that each of the base station systems transmit information which is necessary for enabling the mobile subscriber to receive the multicasting information through respective broadcast channels to each of the mobile subscribers before multicasting information is transmitted.
The method comprises:
(1) sending a multicast request to at least one anchor base station system together with area information for multicasting;
(2) instructing a multicast request from the anchor base station system to at least one branch base station system necessary for covering multicasting area indicated by the area information;
(3) transmitting multicast starting information and spread code to be used for multicasting information to mobile subscribers through respective broadcast channels of the anchor base station system and the branch base station system; and
(4) transmitting multicasting information from the anchor base station system and the branch base station system after proper time interval, during which the mobile subscribers have been ready for receiving multicasting information, has been elapsed.
The multicast communication method set forth above, wherein the anchor base station system uses a diversity hand-over trunk for establishing links to the branch base station systems.
The multicast communication method set forth above, wherein the method further comprises receiving multicasting information, at the mobile subscriber, transmitted from a plurality of base station systems, and performing a RAKE combine process for respective received signals.
In another aspect, the method comprises:
(1) sending a multicast request to at least one anchor base station system together with area information for multicasting;
(2) instructing a multicast request and link establishment request from the anchor base station system to at least one branch base station system necessary for covering multicasting area indicated by the area information;
(3) sending back information of link establishment completion and information of decided spread code for multicasting information at each base station system to the anchor base station system from the branch base station system,
(4) transmitting multicast starting information together with a list indicating base station systems transmitting identical multicasting information and spread code to be used for multicasting information to mobile subscribers through respective broadcast channels of the anchor base station system and the branch base station system; and
(5) transmitting multicasting information from the anchor base station system and the branch base station system after proper time interval, during which the mobile subscribers have been ready for receiving multicasting information, has been elapsed.
The multicast communication method in a CDMA mobile communication system set forth above, wherein the method further comprises:
(6) detecting, at the mobile subscriber, receivable base station systems listed in the list transmitted through the broadcast channel, and preparing reception of multicasting information by setting spread codes indicated in said list; and
(7) receiving multicasting information, at the mobile subscriber, transmitted from a plurality of base station systems, and performing a RAKE combine process for respective received signals.